In order to get minimal distortions caused by misalignment of object and image planes, the following procedure gives best results:
- Point the camera to where the negative is going to be
- Put a mirror in place of the negative
- Adjust camera and backlight so that the image of the lens is centered in the shot. Adjusting is easier with LiveView and with mirrorless cameras.
In this shot, the lens is not centered vertically. Horizontal offset is minimal.
In this shot, the lens is centered vertically, horizontal position has shifted.
For perfect alignment, repeat step 3 until satisfied.
For better repeatability, you want a rigid setup and a remote trigger.